Technical Field
The present disclosure relates to methods for allocating radio resources to a transmitting terminal for performing a direct communication transmission over a direct link connection with a receiving terminal. The present disclosure is also providing the transmitting terminal and base station for participating in the methods described herein.
Description of the Related Art
Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g., parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE each subframe is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g., employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block (PRB) is defined as NsymbDL consecutive OFDM symbols in the time domain (e.g., 7 OFDM symbols) and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4 (e.g., 12 subcarriers for a component carrier). In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
One subframe consists of two slots, so that there are 14 OFDM symbols in a subframe when a so-called “normal” CP (cyclic prefix) is used, and 12 OFDM symbols in a subframe when a so-called “extended” CP is used. For sake of terminology, in the following the time-frequency resources equivalent to the same NscRB consecutive subcarriers spanning a full subframe is called a “resource block pair”, or equivalent “RB pair” or “PRB pair”.
The term “component carrier” refers to a combination of several resource blocks in the frequency domain. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Similar assumptions for the component carrier structure apply to later releases too.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g., to fulfill the requirements on IMT-Advanced.
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers (component carriers) are aggregated in order to support wider transmission bandwidths up to 100MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE are in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanism (e.g., barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g., TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells); with carriers of the SCell being the Downlink Secondary Component Carrier (DL SCC) and Uplink Secondary Component Carrier (UL SCC).
The configuration and reconfiguration, as well addition and removal, as of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover).
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as DL anchor carrier'. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
LTE RRC States
LTE is based on only two main states: “RRC_IDLE” and “RRC_CONNECTED”.
In RRC_IDLE the radio is not active, but an ID is assigned and tracked by the network. More specifically, a mobile terminal in RRC_IDLE performs cell selection and reselection—in other words, it decides on which cell to camp. The cell (re)selection process takes into account the priority of each applicable frequency of each applicable Radio Access Technology (RAT), the radio link quality and the cell status (i.e., whether a cell is barred or reserved). An RRC_IDLE mobile terminal monitors a paging channel to detect incoming calls, and also acquires system information. The system information mainly consists of parameters by which the network (E-UTRAN) can control the cell (re)selection process and also how the mobile terminal accesses the network. RRC specifies the control signalling applicable for a mobile terminal in RRC_IDLE, namely paging and system information. The mobile terminal behaviour in RRC_IDLE is specified in more detail, e.g., in TS 36.304, chapter 4 “General description of Idle mode” incorporated herein by reference.
In RRC_CONNECTED the mobile terminal has an active radio operation with contexts in the eNodeB. The E-UTRAN allocates radio resources to the mobile terminal to facilitate the transfer of (unicast) data via shared data channels. To support this operation, the mobile terminal monitors an associated control channel which is used to indicate the dynamic allocation of the shared transmission resources in time and frequency. The mobile terminal provides the network with reports of its buffer status and of the downlink channel quality, as well as neighbouring cell measurement information to enable E-UTRAN to select the most appropriate cell for the mobile terminal. These measurement reports include cells using other frequencies or RATs. The UE also receives system information, consisting mainly of information required to use the transmission channels. To extend its battery lifetime, a UE in RRC_CONNECTED may be configured with a Discontinuous Reception (DRX) cycle. RRC is the protocol by which the E-UTRAN controls the UE behaviour in RRC_CONNECTED.
The various functions of the mobile terminal in RRC Protocol for and including Connected Mode are described in 3GPP TS36.331 in Ch. 4 “Functions”, incorporated herein by reference.
Uplink Access Scheme for LTE
For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, Node B assigns users a unique time/frequency resource for transmitting user data, thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g., a sub-frame of 0.5 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
UL Scheduling Scheme for LTE
The uplink scheme allows for both scheduled access, i.e., controlled by eNB, and contention-based access.
In case of scheduled access, the UE is allocated a certain frequency resource for a certain time (i.e., a time/frequency resource) for uplink data transmission. However, some time/frequency resources can be allocated for contention-based access. Within these time/frequency resources, UEs can transmit without first being scheduled. One scenario where UE is making a contention-based access is for example the random access, i.e., when UE is performing initial access to a cell or for requesting uplink resources.
For the scheduled access Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically the scheduler determines                which UE(s) is (are) allowed to transmit,        which physical channel resources (frequency),        Transport format (Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission        
The allocation information is signaled to the UE via a scheduling grant, sent on the L1/L2 control channel. For simplicity reasons this channel is called uplink grant channel in the following. A scheduling grant message contains at least information which part of the frequency band the UE is allowed to use, the validity period of the grant, and the transport format the UE has to use for the upcoming uplink transmission. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme. Only “per UE” grants are used to grant the right to transmit on the UL-SCH (i.e., there are no “per UE per RB” grants). Therefore the UE needs to distribute the allocated resources among the radio bearers according to some rules. Unlike in HSUPA, there is no UE based transport format selection. The eNB decides the transport format based on some information, e.g., reported scheduling information and QoS info, and UE has to follow the selected transport format. In HSUPA the Node B assigns the maximum uplink resource, and UE selects accordingly the actual transport format for the data transmissions.
Since the scheduling of radio resources is the most important function in a shared channel access network for determining Quality of service, there are a number of requirements that should be fulfilled by the UL scheduling scheme for LTE in order to allow for an efficient QoS management.                Starvation of low priority services should be avoided        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme        The UL reporting should allow fine granular buffer reports (e.g., per radio bearer or per radio bearer group) in order to allow the eNB scheduler to identify for which Radio Bearer/service data is to be sent.        It should be possible to make clear QoS differentiation between services of different users        It should be possible to provide a minimum bit rate per radio bearer        
As can be seen from above list one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregated cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signaled from AGW to eNB as described before. An operator can then allocate a certain amount of its aggregated cell capacity to the aggregated traffic associated with radio bearers of a certain QoS class. The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to.
(Broadcast) System Information Structure
In the 3GPP terminology, (broadcast) system information is also denoted BCCH information, i.e., it denotes the information carried on the Broadcast Control CHannel (being a logical channel) of the radio cell to which the UE is connected (active state) or attached (idle state).
Generally, system information includes a master information block (MIB) and several system information blocks (SIBs). MIB contains control information on each System Information Block. The control information associated to a respective SIB may have the following structure. Respective control information associated to a SIB may indicate the position of the SIB on a transport channel (e.g., position in the time-frequency plane for OFDM radio access, i.e., particular Resource Blocks being assigned for transmission of a respective SIB) on which it is transmitted relative to common timing reference. Further, a repetition period of SIB may be indicated. This repetition period indicates the periodicity at which the respective SIB is transmitted. The control information may also include a timer value for timer-based update mechanism or, alternatively, a value tag for a tag-based update of the SIB information.
The table below shows an overview of the categorization and types of system information blocks in a UMTS legacy system as defined in 3GPP TS 25.331, “Radio Resource Control (RRC)”, version 12.2.0, section 8.1.1, incorporated herein by reference). System information is also defined for LTE systems, and details can be found in TS 36.331 V12.2.0 subclause 6.3.1, incorporated herein by reference.
As will be explained in much more detail in later sections, a Device-to-Device (D2D) communication technology is going to be implemented for LTE-Rel. 12. Among many other things, the 3GPP standardization is currently in the progress of defining the System InformationBlock Type 18 to contain some information related to ProSe Direct Communication and Discovery. The following definition of SIB18 is taken from the currently discussed change request R2-143565 for TS 36.331 capturing agreements so far regarding ProSe, which however is not yet finally decided and is thus to be seen as a mere example.
SystemInformationBlockType18 information element-- ASN1STARTSystemInformationBlockType18-r12 ::= SEQUENCE {commConfig-r12SEQUENCE {-- FFS if the Rx resource pool can be provided by system informationcommSA-RxResourcePoolCommon-r12ProseCommSA-ResourcePool-r12 OPTIONAL, -- Need ORcommIdleTxPool-r12SEQUENCE{commSA-TxResourcePoolCommon-r12ProseCommSA-ResourcePool-r12,-- FFS whether to signal data Tx resources (needed if not always be inferrable from-- the SA Tx resourcescommData-TxResourcePoolCommon-r12ProseCommDataResourcePool-r12 OPTIONAL -- Need OR}OPTIONAL-- Need OR},OPTIONAL,-- Need ORdiscConfig-r12SEQUENCE {discRxResourcePool-r12ProseDiscResourcePool-r12,discIdleTxPool-r12ProseDiscResourcePool-r12OPTIONAL -- Need OR}OPTIONAL,-- Need ORdisclnterFreqList-r12CarrierFreqList-r12OPTIONAL, -- Need ORlateNonCriticalExtensionOCTET STRINGOPTIONAL,...}CarrierFreqList-r12 ::= SEQUENCE (SIZE (1..maxFreq)) OF ARFCN-ValueEUTRA-r9-- ASN1STOP
SystemInformationBlockType18 field descriptionscommIdleTxPoolIndicates the resources by which the UE is allowed to perform directcommunication transmissions while in RRC_IDLE.discInterFreqListIndicates the neighbouring frequencies on which direct discoveryannouncement is supported.discIdleTxPoolIndicates the resources by which the UE is allowed to transmit directdiscovery announcements while in RRC_IDLE.
As apparent from the above system information, the field commldleTxPool, including the sub-fields commSA-TxResourcePoolCommon indicates the common resources from which any of the UEs receiving the SIB18 and still being in idle can use (in a contention-based manner). In other words, the network operator can commonly define radio resources for all of the UEs, which are however only usable while the UE is still in idle. As will be introduced later, these radio resources defined by commldleTxPool are categorized as Mode 2 resources, for being autonomously used by the UEs.
Buffer Status Reporting
The Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of data available for transmission in the UL buffers of the UE. RRC controls BSR reporting by configuring the two timers periodicBSR-Timer and retxBSR-Timer and by, for each logical channel, optionally signalling logicalChannelGroup which allocates the logical channel to an LCG. Further information on buffer status reporting can be found in 3GPP TS 36.321 subclause 5.4.5, incorporated herein by reference.
LTE Device to Device (D2D) Proximity Services (ProSe)
Proximity-based applications and services represent an emerging social-technological trend. The identified areas include services related to commercial services and Public Safety that would be of interest to operators and users. The introduction of a Proximity Services (ProSe) capability in LTE would allow the 3GPP industry to serve this developing market, and will, at the same time, serve the urgent needs of several Public Safety communities that are jointly committed to LTE.
Device-to-Device (D2D) communication is a technology component for LTE-Rel. 12. The Device-to-Device (D2D) communication technology allows D2D as an underlay to the cellular network to increase the spectral efficiency. For example, if the cellular network is LTE, all data carrying physical channels use SC-FDMA for D2D signaling.
D2D Communication in LTE
The D2D communication in LTE is focusing on two areas: Discovery and Communication.
In D2D communication UEs transmit data signals to each other over a direct link using the cellular resources instead of through the base station (BS). D2D users communicate directly while remaining controlled under the BS, i.e., at least when being in coverage of an eNB. Therefore, D2D can improve system performances by reusing cellular resources.
It is assumed that D2D operates in the uplink LTE spectrum (in the case of FDD) or uplink sub-frames of the cell giving coverage (in case of TDD, except when out of coverage). Furthermore, D2D transmission/reception does not use full duplex on a given carrier. From individual UE perspective, on a given carrier D2D signal reception and LTE uplink transmission do not use full duplex, i.e., no simultaneous D2D signal reception and LTE UL transmission is possible.
In D2D communication when one particular UE1 has a role of transmission (transmitting user equipment or transmitting terminal), UE1 sends data, and another UE2 (receiving user equipment) receives it. UE1 and UE2 can change their transmission and reception role. The transmission from UE1 can be received by one or more UEs like UE2.
With respect to the User plane protocols, in the following part of the agreement from D2D communication perspective is given (see also 3GPP TR 36.843 V12.0.0 section 9.2.2, incorporated herein by reference):
PDCP:
                1:M D2D broadcast communication data (i.e., IP packets) should be handled as the normal user-plane data.        Header-compression/decompression in PDCP is applicable for 1:M D2D broadcast communication.                    U-Mode is used for header compression in PDCP for D2D broadcast operation for public safety;                        RLC:                    RLC UM is used for 1:M D2D broadcast communication.            Segmentation and Re-assembly is supported on L2 by RLC UM.            A receiving UE needs to maintain at least one RLC UM entity per transmitting peer UE.            An RLC UM receiver entity does not need to be configured prior to reception of the first RLC UM data unit.                        So far no need has been identified for RLC AM or RLC TM for D2D communication for user plane data transmission.        MAC:                    No HARQ feedback is assumed for 1:M D2D broadcast communication            The receiving UE needs to know a source ID in order to identify the receiver RLC UM entity.            The MAC header comprises a L2 target ID which allows filtering out packets at MAC layer.            The L2 target ID may be a broadcast, group cast or unicast address.                            L2 Groupcast/Unicast: A L2 target ID carried in the MAC header would allow discarding a received RLC UM PDU even before delivering it to the RLC receiver entity.                L2 Broadcast: A receiving UE would process all received RLC PDUs from all transmitters and aim to re-assemble and deliver IP packets to upper layers.                                    MAC sub header contains LCIDs (to differentiate multiple logical channels).            At least Multiplexing/de-multiplexing, priority handling and padding are useful for D2D.Radio Resource Allocation                        
From the perspective of a transmitting UE, a Proximity Services enabled UE (ProSe-enabled UE) can operate in two modes for resource allocation:
Mode 1 refers to the eNB-scheduled resource allocation, where the
UE requests transmission resources from the eNB (or Release-10 relay node), and the eNodeB (or Release-10 relay node) in turn schedules the exact resources used by a UE to transmit direct data and direct control information (e.g., Scheduling Assignment). The UE needs to be RRC_CONNECTED in order to transmit data. In particular, the UE sends a scheduling request (D-SR or Random Access) to the eNB followed by a buffer status report (BSR) in the usual manner (see also following chapter “Transmission procedure for D2D communication”). Based on the BSR, the eNB can determine that the UE has data for a ProSe Direct Communication transmission, and can estimate the resources needed for transmission.
On the other hand, Mode 2 refers to the UE-autonomous resource selection, where a UE on its own selects resources (time and frequency) from resource pools to transmits direct data and direct control information. One resource pool is defined, e.g., by the content of SIB18 (as introduced in a previous section), namely by the field commIdleTxPool, this particular resource pool being broadcast in the cell, and then commonly available for all UEs in the cell still in RRC_Idle state. As an alternative, or in addition, another resource pool can be defined by the eNB and signaled dedicatedly to the UE, namely by using the field commTxResourcePool. Although not yet finally decided, a corresponding ProSe information element is currently being standardized for TS 36.331 according to the change request R2-143565. Correspondingly, the following definition is merely to be seen as an example:
ProseCommConfig information element-- ASN1STARTProseCommConfig-r12 ::= SEQUENCE {-- FFS if delta signalling should be supported for this IE-- FFS if the Rx resource pool can be provided by dedicated signallingcommSA-RxResourcePoolDedicated-r12ProseCommSA-ResourcePool-r12 OPTIONAL, -- Need ORcommTxResourcePool-r12SEQUENCE{commTxPoolUse-r12ENUMERATED {normal, exceptional),commSA-TxResourcePoolDedicated-r12ProseCommSA-ResourcePool-r12,-- FFS whether to signal data Tx resources (needed if not always be inferrable from-- the SA Tx resourcescommData-TxResourcePoolDedicated-r12ProseCommDataResourcePool-r12 OPTIONAL -- NeedOR}OPTIONAL, -- Need OR...}-- ASN1STOP
ProseCommConfig field descriptionscommSA-RxResourcePoolDedicatedIndicates the pool of resources the UE shall monitor in order to receivescheduling assignments for Prose communication.commTxResourcePoolIf included, the UE is allowed to use the indicated resources in normalor exceptional conditions, depending on commTxPoolUse. If notincluded or when the conditions for use are not fulfilled, a UE that wantsto start direct communication involving transmissions has to requestE-UTRAN to assign resources for each individual transmission,as specified in 36.321 [6].commSA-TxResourcePoolDedicatedIndicates TBC.commData-TxResourcePoolDedicatedIndicates TBC.
This ProSeCommConfig information element can be part of a network response, transmitted by the eNB, in response to a corresponding request by the UE that D2D communication is intended. For example, as illustrated in FIG. 16, the UE may transmit a D2D Communication Interest Indication to the eNB, in case the UE wants to perform D2D communication. The D2D Communication Response (e.g., as part of the RRCCommunicationReconfiguration) then could, e.g., include the above-mentioned ProseCommConfig information element.
Furthermore, the preconfigured radio resources available to a UE that is out of coverage of a cell of an eNB for D2D transmission of a SA or data, may also be categorized as Mode 2 resources.
What resource allocation mode a UE is going to use is configurable by the eNB, as explained above. Furthermore, what resource allocation mode a UE is going to use for D2D data communication may also depend on the RRC state, i.e., RRC_IDLE or RRC_CONNECTED, and the coverage state of the UE, i.e., in-coverage, out-of-coverage. A UE is considered in-coverage if it has a serving cell (i.e., the UE is RRC_CONNECTED or is camping on a cell in RRC_IDLE).
According to agreements so far in 3GPP (see change request to TS 36.300 in R2-143672, section on resource allocation) the following rules with respect to the resource allocation mode apply for the UE:                If the UE is out-of-coverage, it can only use Mode 2;        If the UE is in-coverage, it may use Mode 1 if the eNB configures it accordingly;        If the UE is in-coverage, it may use Mode 2 if the eNB configures it accordingly;        When there are no exceptional conditions, UE may change from Mode 1 to Mode 2 or vice-versa only if it is configured by eNB to do so. If the UE is in-coverage, it shall use only the mode indicated by eNB configuration unless one of the exceptional cases occurs;                    The UE considers itself to be in exceptional conditions, e.g., while T311 or T301 is running;            When an exceptional case occurs the UE is allowed to use Mode 2 temporarily even though it was configured to use Mode 1.                        
While being in the coverage area of an E-UTRA cell, the UE shall perform ProSe Direct Communication Transmission on the UL carrier only on the resources assigned by that cell, even if resources of that carrier have been pre-configured, e.g., in UICC (Universal Integrated Circuit Card).
For UEs in RRC_IDLE the eNB may select one of the following options:                The eNB may provide a Mode 2 transmission resource pool in SIB. UEs that are authorized for ProSe Direct Communication use these resources for ProSe Direct Communication in RRC_IDLE;        
The eNB may indicate in SIB that it supports D2D but does not provide resources for ProSe Direct Communication. UEs need to enter RRC_CONNECTED to perform ProSe Direct Communication transmission.
For UEs in RRC CONNECTED:                A UE in RRC_CONNECTED that is authorized to perform ProSe Direct Communication transmission, indicates to the eNB that it wants to perform ProSe Direct Communication transmissions when it needs to perform ProSe Direct Communication transmission;        The eNB validates whether the UE in RRC_CONNECTED is authorized for ProSe Direct Communication transmission using the UE context received from MME;        The eNB may configure a UE in RRC_CONNECTED by dedicated signalling with a Mode 2 resource allocation transmission resource pool that may be used without constraints while the UE is RRC_CONNECTED. Alternatively, the eNB may configure a UE in RRC_CONNECTED by dedicated signalling with a Mode 2 resource allocation transmission resource pool which the UE is allowed to use only in exceptional cases and rely on Mode 1 otherwise.        
This behavior of the UE with respect to the resource allocation is illustrated in a simplified manner according to the state diagrams of FIG. 7 and FIG. 8. FIG. 7 refers to the case where the UE is in the RRC_IDLE state, and differentiates between in-coverage and out-of-coverage. It should be noted that a UE which is in out-of-coverage and in RRC_IDLE can use the Mode 2 resource allocation. There are no exceptional cases at the moment defined for a UE in RRC_IDLE. On the other hand, FIG. 8 refers to the case where the UEs are in the RRC_CONNECTED state, and differentiates between in-coverage and the exceptional case. As apparent, a connected UE being in an exceptional case can use the Mode 2 resource allocation.
FIG. 9 illustrates the use of transmission/reception resources for overlay (LTE) and underlay (D2D) system.
Basically, the eNodeB controls whether UE may apply the Mode 1 or Mode 2 transmission. Once the UE knows its resources where it can transmit (or receive) D2D communication, in the current state-of-the-art, it uses the corresponding resources only for the corresponding transmission/reception. For example, in FIG. 9, the D2D subframes will only be used to receive or transmit the D2D signals. Since the UE as a D2D device would operate in Half Duplex mode, it can either receive or transmit the D2D signals at any point of time. Similarly, the other subframes illustrated in FIG. 9 can be used for LTE (overlay) transmissions and/or reception.
Transmission Procedure for D2D Communication
The D2D data transmission procedure differs depending on the resource allocation mode. As described above for Mode 1, the eNB explicitly schedules the resources for the Scheduling Assignment and the D2D data communication after a corresponding request from the UE. Particularly, the UE may be informed by the eNB that D2D communication is generally allowed, but that no Mode 2 resources (i.e., resource pool) are provided; this may be done, e.g., with the exchange of the D2D communication Interest Indication by the UE and the corresponding response D2D Communication Response as illustrated in FIG. 16, where the corresponding exemplary ProseCommConfig information element mentioned above would not include the commTxResourcePool meaning that a UE that wants to start direct communication involving transmissions has to request E-UTRAN to assign resources for each individual transmission. Thus, in such a case, the UE has to request the resources for each individual transmission, and in the following the different steps of the request/grant procedure are exemplarily listed for this Mode 1 resource allocation:                Step 1: UE sends SR (Scheduling Request) to eNB via PUCCH;        Step 2: eNB grants UL resource (for UE to send BSR) via PDCCH, scrambled by C-RNTI;        Step 3: UE sends D2D BSR indicating the buffer status via PUSCH;        Step 4: eNB grants D2D resource (for UE to send data) via PDCCH, scrambled by D2D-RNTI.        Step 5: D2D Tx UE transmits SA/D2D data according to grant received in step 4.        
A Scheduling Assignment (SA) is a compact (low-payload) message containing control information, e.g., pointer(s) to time-frequency resources for the corresponding D2D data transmissions. The content of the SA is basically in accordance with the grant received in Step 4 above. The exact details of the D2D grant and SA content are not fixed yet but as a working assumption for SA content the following agreements were achieved                Frequency resource is indicated by Rel-8 UL Type 0 resource allocation (5-13 bits depending on System BW)        1 bit frequency hopping indicator (as per Rel-8)                    some reinterpretation of the indexing is to be defined so that hopping does not use PRBs outside the configured resource pool for mode 2.                        Only single-cluster resource allocations are valid                    this implies that if there are gaps in the resource pool in the frequency domain, a resource allocation shall not straddle a gap                        No RV indicator in SARV pattern for data: {0, 2, 3, 1}.        
On the other hand, for Mode 2 resource allocation, above steps 1-4 are basically not necessary, and the UE autonomously selects resources for the SA and D2D data transmission from the transmission resource pool(s) configured and provided by the eNB.
FIG. 10 exemplarily illustrates the transmission of the Scheduling Assignment and the D2D data for two UEs, UE-A and UE-B, where the resources for sending the scheduling assignments are periodic, and the resources used for the D2D data transmission are indicated by the corresponding Scheduling Assignment.
Resource Pool for Scheduling Assignment and D2D Data
The resource pool for Scheduling Assignment (SA) and D2D data when the UE is out-of-coverage can be configured as below:                The resource pool used for reception of SA is pre-configured.        The resource pool used for transmission of SA is pre-configured.        The resource pool used for reception of D2D data is pre-configured.        The resource pool used for transmission of D2D data is pre-configured.        
The resource pool for Scheduling Assignment (SA) when the UE is in-coverage can be configured as below:                The resource pool used for reception of SA is configured by the eNB via RRC, in dedicated or broadcast signalling.        The resource pool used for transmission of SA is configured by the eNB via RRC if Mode 2 resource allocation is used        The SA resource pool used for transmission is not known to the UE if Mode 1 resource allocation is used.        The eNB schedules the specific resource(s) to use for Scheduling Assignment transmission if Mode 1 resource allocation is used. The specific resource assigned by the eNB is within the resource pool for reception of Scheduling Assignment that is provided to the UE.UE Coverage states for D2D        
As already mentioned before (see, e.g., FIG. 7 and FIG. 8), the resource allocation method for D2D communication depends apart from the RRC state, i.e., RRC_IDLE and RRC_CONNECTED, also on the coverage state of the UE, i.e., in-coverage, out-of-coverage. A UE is considered in-coverage if it has a serving cell (i.e., the UE is RRC_CONNECTED or is camping on a cell in RRC_IDLE).
The two coverage states mentioned so far, i.e., in-coverage (IC) and out-of-coverage (OOC), are further distinguished into sub-states for D2D. FIG. 11 shows the four different states a D2D UE can be associated to, which can be summarized as follows:                State 1: UE1 has uplink and downlink coverage. In this state the network controls each D2D communication session. Furthermore, the network configures whether UE1 should use resource allocation Mode 1 or Mode 2.        State 2: UE2 has downlink but no uplink coverage, i.e., only DL coverage. The network broadcasts a (contention-based) resource pool. In this state the transmitting UE selects the resources used for SA and data from a resource pool configured by the network; resource allocation is only possible according to Mode 2 for D2D communication in such a state.        State 3: Since UE3 has no uplink and downlink coverage, the UE3 is strictly speaking already considered as out-of-coverage (OOC). However, UE3 is in the coverage of some UEs which are themselves (e.g., UE1) in the coverage of the cell, i.e., those UEs can be also referred as CP-relay UEs, therefore the area of the state 3 UEs in FIG. 11 can be denoted as CP UE-relay coverage area. UEs in this state 3 are also referred to as OOC-state 3 UEs. In this state the UEs receive some cell specific information which is sent by the eNB (SIB) and forwarded by the CP UE-relay UEs in the coverage of the cell via PD2DSCH to the OOC-state 3 UEs. A (contention-based) network-controlled resource pool is signalled by PD2DSCH.        State 4: UE4 is out of coverage and does not receive PD2DSCH from other UEs which are in the coverage of a cell. In this state, which is also referred to as state 4 OOC, the transmitting UE selects the resources used for the data transmission from a pre-configured pool of resources.        
The reason to distinguish between state 3 OOC and state 4 OOC is mainly to avoid potentially strong interference between D2D transmissions from out-of coverage devices and legacy E-UTRA transmissions. In general D2D-capable UEs will have preconfigured resource pool(s) for transmission of D2D SAs and data for use while out of coverage. If these out-of-coverage UEs transmit on these preconfigured resource pools near cell boundaries, then, interference between the D2D transmissions and in-coverage legacy transmissions could have a negative impact on communications within the cell. If D2D-enabled UEs within coverage forwarded the D2D resource pool configuration to those out-of-coverage devices near the cell boundary, then, the out-of-coverage UEs could restrict their transmissions to the resources specified by the eNode B and therefore minimise interference with legacy transmissions in coverage. Thus, RAN1 introduced a mechanism where in-coverage UEs are forwarding resource pool information and other D2D related configurations to those devices just outside the coverage area (state 3 UEs).
The Physical D2D synchronization channel (PD2DSCH) is used to carry this information about in-coverage D2D resource pools to the UEs in network proximity, so that resource pools within network proximity are aligned. The detailed content of the PD2DSCH is not finalized yet though.
D2D Discovery
ProSe (Proximity based Services) Direct Discovery is defined as the procedure used by the ProSe-enabled UE to discover other ProSe-enabled UE(s) in its proximity using E-UTRA direct radio signals via the PC5 interface. FIG. 12 schematically illustrates a PC5 interface for device-to-device direct discovery.
The upper layer handles authorization for announcement and monitoring of discovery information. For this purpose, UEs have to exchange predefined signals, referred to as discovery signals. By checking discovery signals periodically, a UE maintains a list of proximity UEs in order to establish a communication link when it is needed. Discovery signals should be detected reliably, even in low Signal-to-Noise Ratio (SNR) environments. To allow discovery signals to be transmitted periodically, resources for discovery signals should be assigned.
There are two types of ProSe Direct Discovery: open and restricted. Open is the case where there is no explicit permission that is needed from the UE being discovered, whereas restricted discovery only takes place with explicit permission from the UE that is being discovered.
ProSe Direct Discovery can be a standalone service enabler in a discovering UE, which enables the discovering UE to use information from a discovered UE for certain applications. As an example, the information transmitted in ProSe Direct Discovery may be “find a taxi nearby”, “find me a coffee shop”, “find me the nearest police station” and the like. Through ProSe Direct Discovery a discovery UE can retrieve needed information. Additionally, depending on the information obtained, ProSe Direct Discovery can be used for subsequent actions in the telecommunication system, such as, initiating a ProSe Direct Communication.
ProSe Direct Discovery Models
ProSe Direct Discovery is based on several discovery models. An overview is given in the following. The models for ProSe Direct Discovery are defined in more detail in 3GPP TS 23.303 V12.1.0, section 5.3 which is enclosed herein by reference.
Model A (“I am here”)
Model A is also indicated as “I am here”, since the announcing UE broadcasts information about itself, such as its ProSe Application Identities or ProSe UE Identities in the discovery message, thereby identifying itself and communicating to the other parties of the communication system that it is available.
According to Model A, two roles for ProSe-enabled UEs that are participating in ProSe Direct Discovery are defined. ProSe-enabled UE can have the function of Announcing UE and Monitoring UE. An announcing UE announces certain information that could be used by UEs in the proximity that have permission to discover. A Monitoring UE monitors certain information of interest in the proximity of announcing UEs.
In this Model A the announcing UE broadcasts discovery messages at pre-defined discovery intervals, and the monitoring UEs that are interested in these messages read them and process them.
Mode B (“who is there?”/“are you there?”)
This model defines two roles for the ProSe-enabled UEs that are participating in ProSe Direct Discovery:                Discoverer UE: The UE transmits a request containing certain information about what it is interested to discover;        Discoveree UE: The UE that receives the request message can respond with some information related to the discoverer's request.        
Model B is equivalent to “who is there/are you there” since the discoverer UE transmits information about other UEs that would like to receive responses from. The transmitted information can be, for example, about a ProSe Application Identity corresponding to a group. The members of the group can respond to said transmitted information.
According to this Model B, two roles for the ProSe-enabled UEs that are participating in ProSe Direct Discovery are defined: discoverer UE and discoveree UE. The discoverer UE transmits a request containing certain information about what it is interested to discover. On the other hand, the discoveree UE receives the request message and can respond with some information related to the discoverer UE's request.
The content of the discovery information is transparent to the Access Stratum (AS), which does not know the content of discovery information. Thus, no distinction is made in the Access Stratum between the various ProSe Direct Discovery models and types of ProSe Direct Discovery. The ProSe Protocol ensures that it delivers only valid discovery information to AS for announcement.
The UE can participate in announcing and monitoring of discovery information in both RRC_IDLE and RRC_CONNECTED state as per eNB configuration. The UE announces and monitors its discovery information subject to the half-duplex constraints.
Types of Discovery
FIG. 13 illustrates a diagram showing the IDLE and CONNECTED mode in the reception of discovery resources in D2D communication, and with regard to the resource allocation procedure.
D2D communication may either be network-controlled, where the operator manages the switching between direct transmissions (D2D) and conventional cellular links, or the direct links may be managed by the devices without operator control. D2D allows combining infrastructure-mode and ad-hoc communication.
Generally, device discovery is needed periodically. Further, D2D devices utilize a discovery message signalling protocol to perform device discovery. For example, a D2D-enabled UE can transmit its discovery message, and another D2D-enabled UE receives this discovery message and can use the information to establish a communication link. An advantage of a hybrid network is that, if D2D devices are also in communication range of network infrastructure, network entities, like eNB, can additionally assist in the transmission or configuration of discovery messages. Coordination/control by the eNB in the transmission or configuration of discovery messages is also important to ensure that D2D messaging does not create interference with the cellular traffic controlled by the eNB. Additionally, even if some of the devices are outside of the network coverage range, in-coverage devices can assist in the ad-hoc discovery protocol.
At least the following two types of discovery procedures are defined for the purpose of terminology definition used further in the description.                Type 1: A resource allocation procedure where resources for announcing of discovery information are allocated on a non-UE-specific basis, further characterized by:                    The eNB provides the UE(s) with the resource pool configuration used for announcing discovery information. The configuration may be signalled in SIB.            The UE autonomously selects radio resource(s) from the indicated resource pool and announces discovery information.            The UE can announce discovery information on a randomly selected discovery resource during each discovery period.                        Type 2: A resource allocation procedure where resources for announcing of discovery information are allocated on a per UE specific basis, further characterized by:                    The UE in RRC_CONNECTED may request resource(s) for announcing of discovery information from the eNB via RRC.                            The eNB assigns resource(s) via RRC.                                    The resources are allocated within the resource pool that is configured in UEs for monitoring.The resources are, according to the Type 2 procedure, for example allocated semi-persistently for discovery signal transmission.                        
In the case UEs are in RRC_IDLE modus, the eNB may select one of the following options:                The eNB may provide a Type 1 resource pool for discovery information announcement in SIB. UEs that are authorized for ProSe Direct Discovery use these resources for announcing discovery information in RRC_IDLE.        The eNB may indicate in SIB that it supports D2D but does not provide resources for discovery information announcement. UEs need to enter RRC Connected in order to request D2D resources for discovery information announcement.        
For UEs in RRC_CONNECTED status, a UE authorized to perform ProSe Direct Discovery announcement indicates to the eNB that it wants to perform D2D discovery announcement. Then, the eNB validates whether the UE is authorized for ProSe Direct Discovery announcement using the UE context received from MME. The eNB may configure the UE to use a Type 1 resource pool or dedicated Type 2 resources for discovery information announcement via dedicated RRC signalling (or no resource). The resources allocated by the eNB are valid until a) the eNB de-configures the resource (s) by RRC signalling or b) the UE enters IDLE.
Receiving UEs in RRC_IDLE and RRC_CONNECTED monitor both Type 1 and Type 2 discovery resource pools as authorised. The eNB provides the resource pool configuration used for discovery information monitoring in SIB. The SIB may contain discovery resources used for announcing in neighbour cells as well.
Radio Protocol Architecture
FIG. 14 schematically illustrates a Radio Protocol Stack (AS) for ProSe Direct Discovery.
The AS layer interfaces with upper layer (ProSe Protocol). Accordingly, the MAC layer receives the discovery information from the upper layer (ProSe Protocol). In this context, the IP layer is not used for transmitting the discovery information. Further, the AS layer has a scheduling function, according to which the MAC layer determines the radio resource to be used for announcing the discovery information received from upper layer. In addition, the AS layer has the function of generating Discovery PDU, according to which the MAC layer builds the MAC PDU carrying the discovery information and sends the MAC PDU to the physical layer for transmission in the determined radio resource. No MAC header is added.
In the UE, the RRC protocol informs the discovery resource pools to MAC. RRC also informs allocated Type 2 resource for transmission to MAC. There is no need for a MAC header. The MAC header for discovery does not comprise any fields based on which filtering on Layer 2 could be performed. Discovery message filtering at the MAC level does not seem to save processing or power compared to performing filtering at the upper layers based on the ProSe UE-and/or ProSe Application ID. The MAC receiver forwards all received discovery messages to upper layers. MAC will deliver only correctly received messages to upper layers.
In the following it is assumed that L1 (PHY) indicates to MAC whether a discovery messages has been received correctly. Further, it is assumed that the Upper Layers guarantee to deliver only valid discovery information to the Access Stratum.
D2D Synchronization
The main task of synchronization is to enable the receivers to acquire a time and frequency reference. Such reference may be exploited for at least two goals: 1) aligning the receiver window and frequency correction when detecting D2D channels and 2) aligning the transmitter timing and parameters when transmitting D2D channels. The following channels have been defined in 3GPP so far for the purpose of synchronization:                D2DSS D2D Synchronization Signal        PD2DSCH Physical D2D Synchronization Channel        PD2DSS Primary D2D Synchronization Signal        SD2DSS Secondary D2D Synchronization Signal        
Furthermore, the following terminology with respect to synchronization was agreed in 3GPP, and will be exemplarily used in the rest of the application.                D2D Synchronization Source: A node that at least transmits a D2D synchronization signal. A D2D synchronization source can be basically an eNB or a D2D UE.        D2D Synchronization Signal: A signal from which a UE can obtain timing and frequency synchronization        
D2D synchronization could be seen as a procedure which is similar to LTE cell search. In order to allow both NW control and efficient synchronization for partial/outside coverage scenarios, the following receiver and transmitter synchronization procedures are currently under discussion within 3GPP:
Receiver Synchronization
The ProSe enabled UE regularly searches for LTE cells (according to LTE mobility procedures) and for D2DSS/PD2DSCH transmitted by Synchronization Source (SS) UEs.
If any suitable cell is found, the UE camps on it and follows the cell synchronization (according to LTE legacy procedures).
If any suitable D2DSS/PD2DSCH transmitted by SS UEs are found, the UE synchronizes its receiver to all incoming D2DSS/PD2DSCH (subject to UE capabilities) and monitors them for incoming connections (Scheduling Assignments). It should be noted that the D2DSS transmitted by a D2D Synchronization Source that is an eNodeB shall be the Rel-8 PSS/SSS (Primary and Secondary Synchronization Signals). D2D Synchronization Sources which are eNodeBs have a higher priority than D2D Synchronization Sources which are UEs.
Transmitter Synchronization
The ProSe enabled UE regularly searches for LTE cells (according to LTE mobility procedures) and for D2DSS/PD2DSCH transmitted by SS UEs;
If any suitable cell is found, the UE camps on it and follows the cell synchronization for D2D signals transmission. In such a case, the network may configure the UE to transmit D2DSS/PD2DSCH following the cell synchronization.
If no suitable cell is found, the UE verifies if any of the incoming D2DSS/PD2DSCH may be relayed further (i.e., the max hop count has not been reac hed), then (a) if an incoming D2DSS/PD2DSCH that may be relayed further is found, the UE adapts its transmitter synchronization to it and transmits D2DSS/PD2DSCH accordingly; or (b) if an incoming D2DSS/PD2DSCH that may be relayed further is NOT found, the UE acts as independent synchronization source and transmit D2DSS/PD2DSCH according to any internal synchronization reference.
Further details on the synchronization procedure for D2D can be found in TR 36.843 V12.0.1, clause 7, incorporated herein by reference.
Cell Selection and RRC Connection Establishment
FIG. 15 illustrates in a simplified and exemplary manner the prior art message exchange between a UE and an eNB for selecting a cell and establishing an RRC connection. Cell selection in step 2 is based, e.g., on 3GPP TS 36.304, e.g., chapter 5.2.3 of V12.1.0, incorporated herein by reference. A UE that is not camping on any WAN (Wide Area Network, e.g., LTE) cell is considered Out of Coverage. Cell Camping may be based on the Cell Selection Criteria/Process as defined in Chapter 5.2.3 of 3GPP TS 36.304 V12.1.0. Therefore, before completion of step 2, the UE is generally considered in Out of Coverage (OOC). Once the Cell Selection is successful and the UE is camped (on a Suitable Cell or on an Acceptable cell), it is in RRC Idle state. UE Continues to be in RRC Idle state until step 7, i.e., until it receives RRCConnectionSetup message from the network, after which it changes to RRC Connected state.